[w3ctag/design-reviews] Request for review: CSS tree-scoped at-rule names and references (for @font-face, @keyframes, etc.) (#659)

Ya ya yawm TAG!

I'm requesting a TAG review of CSS tree-scoped at-rule names and references (for @font-face, @keyframes, etc.).

It has been a long standing issue on how name-defining at-rules (@font-face etc.) should be handled across shadow tree boundaries. The [existing behaviors](https://wiki.csswg.org/spec/css-scoping) either break shadow tree encapsulation, or do not allow such at-rules in shadow trees at all. They are non-interoperable between browsers, and even inconsistent in the same browser between different rules. 

Following a [recent CSSWG resolution](https://github.com/w3c/csswg-drafts/issues/1995#issuecomment-848941922), a new and reasonable behavior has been proposed. Chromium is planning to implement this new behavior, starting with the [@counter-style rule](https://chromestatus.com/feature/5716198446596096). So I'm requesting a TAG review of this new spec.

  - Explainer (I think the examples in the spec are good enough to explain the new behavior):
     - https://drafts.csswg.org/css-scoping/#example-f1503361

     - https://drafts.csswg.org/css-scoping/#example-ee72cb37

  - Specification URL: https://drafts.csswg.org/css-scoping/#shadow-names

  - Tests:
     - https://wpt.fyi/results/css/css-counter-styles/counter-style-access-from-shadow-dom.html

     - https://wpt.fyi/results/css/css-counter-styles/counter-styles-override-in-shadow-dom.html

     - https://wpt.fyi/results/css/css-counter-styles/counter-style-fallbacks-in-shadow-dom.html

     - https://wpt.fyi/results/css/css-counter-styles/counter-styles-shadow-dom-part.html

  - Security and Privacy self-review: I do not expect security/privacy risks
  - Primary contacts (and their relationship to the specification):
      - Xiaocheng Hu (@xiaochengh), Google, Implementer
      - Tab Atkins (@tabatkins), Google, Spec Editor
  - Organization(s)/project(s) driving the specification: CSSWG
  - Key pieces of existing multi-stakeholder review or discussion of this specification:
     - https://github.com/w3c/csswg-drafts/issues/1995

  - External status/issue trackers for this specification (publicly visible, e.g. Chrome Status):
     -  https://chromestatus.com/feature/5716198446596096 (for @counter-style only)

Further details:

  - [X] I have reviewed the TAG's [Web Platform Design Principles](https://w3ctag.github.io/design-principles/)
  - Relevant time constraints or deadlines: August 12, 2021
  - The group where the work on this specification is currently being done: CSSWG
  - The group where standardization of this work is intended to be done (if current group is a community group or other incubation venue): CSSWG
  - Major unresolved issues with or opposition to this specification: I'm not aware of any
  - This work is being funded by: Google

We'd prefer the TAG provide feedback as (please delete all but the desired option):

  💬 leave review feedback as a **comment in this issue** and @-notify @xiaochengh


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